There are various types of logic circuit technologies in conventional use. Two of the most common types are TTL and ECL. Designers choose among the various types of circuits based upon the availability of devices already designed, the desired speed of the circuit, the power consumption limitations of the circuit and other well known factors.
Often it is desirable to use more than one type of logic circuit technology in a given digital circuit. For instance, a designer may use a TTL data storage device in a circuit comprised of ECL logic. The digital signal produced by the TTL device may switch five volts from high to low while the ECL logic circuit operates with a voltage swing of a few hundred millivolts. Thus there is a need for circuitry translating the signal from the TTL device to a level compatible with the ECL logic.
Conventionally in the above TTL to ECL example, the translation is accomplished by using a specially adapted ECL translation circuit which receives the TTL signal and produces an ECL output. The TTL signal is received by a TTL register from which the prior art translation circuit detects the level of the TTL signal, then generates an ECL output. Thus a delay is caused by waiting for the data to latch in the TTL register before the translation circuit can begin generating the ECL output.
The prior art ECL translation circuit further requires supply voltage levels different from the TTL circuitry with which it is associated, such as the base reference voltage for the ECL current switching transistors. Accordingly, the prior art ECL translation circuit is complicated to implement. Also, because of the large voltage difference between TTL and ECL circuits, complicated circuitry such as clamping diodes on the base of the ECL current switching transistors are necessary to prevent overdriving the translation circuit.
Accordingly, there is a need for digital signal translation circuit which is faster and less complicated than prior art translators.